Extending GSUB of existing font using features file

Roel Nieskens

I have an existing font (.ttf or as dumped .ttx) that I want to extend with additional GSUB rules. I wanted to work with an AFDKO feature file, so I created a UFO, and filled a features.fea with my desired substitutions. The UFO has dummy placeholder glyphs as the existing font glyphs should be used. The UFO compiles nicely and the GSUB rules are working.

I am now looking for a way to properly merge this with the existing font, which already has a GSUB table.

What I tried:

• Dumping the new GSUB table and use ttx to merge it in the existing font (doesn't work, existing GSUB gets overwritten instead of merged)
• Manually transplanting the new GSUB table, updating indexes etc. with a simple script. Works, but feels hacky.

What I think might work but feels too nasty to actually do:

• Dump the existing .ttf to UFO, add new GSUB rules, compile UFO back to .ttf.

Any advice on a cleaner way?

Georg Seifert

> Manually transplanting the new GSUB table, updating indexes etc. with a simple script. Works, but feels hacky.

This seems to be the safest route. Decompiling the existing GPOS to fea might break things.

André G. Isaak

This sounds like a job for DTL OTMaster.

André

Kent Lew

I suppose you could do what you suggest and dump the existing.ttf to UFO, add new GSUB rules, compile UFO back to .ttf . . . but then, to get around what Georg cautions (which I agree with), you could dump the new .ttf to .ttx to get the newly compiled, merged GSUB table and insert that back into the original .ttf.

That way, you utilize a compiler to get the new updated GSUB, instead of manual work, but you maintain everything else from the original intact.

Roel Nieskens

Thanks for the suggestions! I think I'll stick to the custom script for now.

@Kent Lew That's sounds like a relatively clean way to do something dirty. Will keep that in mind as a second option!

Khaled Hosny

FontForge can merge a feature file to a font extending existing GSUB/GPOS tables.